Transmission thump/shunt

[tintin]

Rather worringly, I had a fairly serious sounding thump from the transmission this evening in my (D2) S8 - this happened first when travelling, when I moved the selector into S to get past some slower vehicles, and since then it's happened three or four times when pulling off from stationary. I've not had any previous transmission/gearbox shunting/thumping issues, and not done anything major to the car recently (other than changing to winter tyres )

It's sufficiently serious that I'm a bit concerned to pull off in it again at all at the moment - any thoughts, both on cause and how to deal with it? I don't have a Vagcom.